1. Field of the Invention:
The present invention relates to an air-fuel mixture intake construction having a two-barrel or duplex carburetor for internal combustion engines, the intake construction including an induction passage for producing air-fuel mixture swirls in combustion chambers on intake stroke.
2. Prior Art:
There have been known and practiced two-barrel carburetors comprising a primary intake system for supplying an air-fuel mixture under a full range of engine loads and a secondary intake system for supplying an air-fuel mixture under medium and high engine loads to promote fuel atomization under low engine loads and prevent power reduction under the medium and high engine loads.
Many proposals have been made to cause flames to spread at higher speeds in combustion chambers in such an internal combustion engine. One such proposed structure comprises an induction passage or pipe for ejecting air or an air-fuel mixture into the combustion chamber at a high speed to form swirls therein. The induction passage opens at one end into a secondary intake passage upstream of a secondary throttle valve so as to introduce therein air or an air-fuel mixture from the secondary intake passage. There has been a tendency, however, for fuel in liquid form flowing down the secondary intake passage to find its way into the induction passage, from which such liquid fuel enters the combustion chamber. Therefore, the air-fuel mixture in the combustion chamber becomes too rich with the result that unburned components or hydrocarbons will be produced and discharged. Furthermore, as the primary and secondary throttle valves open more widely, the amount of air or an air-fuel mixture ejected through the induction passage is decreased, resulting in a reduction in the rate at which frames spread in the combustion chamber when the engine operates under medium and high loads. This also allows unburned hydrocarbons to be discharged.
Multiple-cylinder engines include an intake manifold for distribution an air-fuel mixture from a single carburetor among the multiple cylinders. The intake manifold is made of cast metal and hence has its branch surfaces roughened to various degrees. The manifold branches exhibit different resistances to the flow of an air-fuel mixture passing therethrough. Such different resistances are partly attributable to uneven distribution of air-fuel mixture among the engine cylinders especially when the amount of air-fuel mixture required is small in an idling or low-load mode of operation of the engine.
Air-fuel mixture intake constructions with primary and secondary intake systems have a secondary slow fuel supply passage for supplying fuel to enable smooth engine operation when the secondary intake system comes into operation. During normal engine operation, the primary intake system is mainly in operation, and the secondary slow fuel passage tends to be subjected to vapor lock, with the consequence that the engine will undergo hesitation and impair drivability. To cope with this problem, various proposals have been made in which a portion of fuel in the secondary slow fuel passage is drawn at all times into the primary intake passage or introduced into the secondary intake passage downstream of the secondary throttle valve. The amount of fuel which is quite small is however rendered unstable or varied under the influences of pulsating engine operation and high negative pressure in the intake passages while the engine operates under low loads, a disadvantage which prohibits smooth engine operation.